Superfluid-Mott glass quantum multicritical point on a percolating lattice

We employ large-scale Monte Carlo simulations to study a particle-hole symmetric site-diluted quantum rotor model in two dimensions. The ground state phase diagram of this system features two distinct quantum phase transitions between the superfluid and the insulating (Mott glass) phases. They are separated by a multicritical point. The generic transition for dilutions below the lattice percolation threshold is driven by quantum fluctuations while the transition across the percolation threshold is due to the geometric fluctuations of the lattice. We determine the location of the multicritical point between these two transitions and find its critical behavior. The multicritical exponents read z = 1.72(2), beta/nu = 0.41(2), and gamma/nu = 2.90(5). We compare our results to other quantum phase transitions in disordered systems, and we discuss experiments.